Thermodynamics of efficient, simple-cycle combustion engines

Engines convert energy in a resource to work---for instance, to propel vehicles and drive electric generators. In 2004, the transportation and electric power sectors account for nearly 60% of global energy consumption and the attendant carbon dioxide emissions. Simple-cycle combustion engines dominate the transportation sector and underpin the operation of many power plants. This dissertation establishes the thermodynamic requirements for this class of engines to be efficient, thus providing guidance for future development of high-efficiency combustion engines with low greenhouse-gas emissions.; The dissertation first considers efficiency maximization for an adiabatic, homogeneous piston engine. In thermodynamic terms, the optimal solution reduces to carrying out combustion at the highest possible internal energy state, to minimize the entropy change from reactants to products at constant internal energy U and volume V. This strategy remains operative when non-adiabatic, non-homogeneous piston and gas turbine engines are considered. The extreme state principle is hence shown to govern the optimal operation of all simple-cycle combustion engines.; The optimal combustion process is also separable from the preceding reactant preparation and the subsequent work extraction steps. Work extraction is optimized when the combustion products expand to the environmental state before discharge. However, this optimality condition is mismatched to combustion. Reactant compression is the most effective way to alleviate the mismatch. Other reactant preparation strategies considered---heating, cooling, dilution with exhaust and excess air---affect overall cycle efficiency to a lesser extent. At fixed compression ratio, the potential for efficiency improvement via piston motion optimization is small, estimated at 2--3% gain compared to slider-crank motion.; The dissertation concludes by considering a generalized piston engine which is controlled by work, heat, and matter transfers, and which destroys exergy due to combustion, engine cooling and blowby. The minimum entropy generation solution for this system still reduces to minimizing the entropy change from reactants to products at constant U, V, and atomic composition. However, active control of heat and matter transfers is more challenging compared to work. This disparity and its thermodynamic implications on optimal reactive engine operation are outlined.